This invention relates to microlithography as performed using a charged particle beam (e.g., electron beam or ion beam) as used in the manufacture of semiconductor integrated circuits, displays, and the like. More specifically, the invention pertains to reticle xe2x80x9cblanksxe2x80x9d as used to make pattern-defining reticles for use in such microlithography, and to methods for manufacturing such reticle blanks.
The dramatic progressive reduction in the sizes of circuit elements in integrated circuits that has occurred in recent years has created a need for image resolution better than that obtainable using optical microlithography systems that are limited by the diffraction of light. This has led to the ongoing development of microlithography (projection-exposure) systems that, instead of using light, employ an X-ray beam or a charged particle beam such as electron beam or an ion beam.
Current charged-particle-beam (CPB) systems include electron-beam pattern-drawing (xe2x80x9cdirect-writexe2x80x9d) systems in which an electron beam is used to form a pattern directly (i.e., without having to project a pattern onto the wafer). Because of the current ability to stop an electron beam down to a spot diameter of a few xc3x85ngstroms, high-resolution sub-micron patterns can be formed in this way. A major drawback of direct-write systems is the fact that the pattern is drawn element-by-element and line-by-line (i.e., by xe2x80x9cdirect writingxe2x80x9d). To draw a finer element, the electron beam simply is stopped down further to a smaller spot diameter. However, reducing the spot diameter increases the amount of time (xe2x80x9cwriting timexe2x80x9d) that must be expended to draw the entire pattern. Increasing the writing time correspondingly reduces throughput and increases device-production costs. Consequently, direct-write systems are impractical for mass production of chip-containing wafers.
The shortcomings of direct-write systems has motivated a large amount of development effort currently being directed to the development of a practical CPB microlithography system that projects (with demagnification) a pattern image from a xe2x80x9creticlexe2x80x9d or xe2x80x9cmaskxe2x80x9d to the wafer. Such systems are termed xe2x80x9creduced-image projection-exposurexe2x80x9d CPB microlithography systems, in which a reticle defining the prescribed pattern is illuminated by a charged particle beam (e.g., electron beam), and a reduced (demagnified) image of the pattern located within the range of illumination is transferred onto the wafer by a projection lens.
By xe2x80x9cdemagnificationxe2x80x9d is meant that the image as formed on the wafer is smaller (usually by an integer factor such as xc2xc or ⅕) than the corresponding illuminated region on the reticle.
As noted above, the pattern is defined on a xe2x80x9creticlexe2x80x9d (sometimes termed a xe2x80x9cmask,xe2x80x9d but generally herein the term xe2x80x9creticlexe2x80x9d is used). Two general types of reticles are known. A first type is termed a xe2x80x9cscattering-membranexe2x80x9d reticle 21, a portion of which is shown schematically in FIG. 7(a). The scattering-membrane reticle comprises a reticle membrane 22 on which regions 24 are formed. The regions 24 are of a substance that scatters particles of a charged particle beam incident from above. The reticle membrane 22 is sufficiently thin to be transmissive to particles of the incident beam and thus exhibit essentially no scattering. The regions 24, in combination with the transmissive membrane 22, define the elements of the pattern. A second type of reticle is termed a xe2x80x9cscattering-stencilxe2x80x9d reticle 31, a portion of which is shown schematically in FIG. 7(b). The scattering-stencil reticle comprises a reticle membrane 32 (typically made of silicon) having a thickness (approximately 2 xcexcm) sufficient to scatter particles of the incident beam. The membrane 32 defines through-holes 34 that are transmissive to particles of the incident beam. The through-holes 34, in combination with the membrane 34, define the elements of the pattern.
In CPB microlithography, it currently is impossible to project an entire pattern in one xe2x80x9cshot.xe2x80x9d As a result, the pattern as defined on the reticle is divided or xe2x80x9csegmentedxe2x80x9d into multiple small portions termed xe2x80x9csubfieldsxe2x80x9d 22a, 32a each defining a respective portion of the overall pattern and each containing a respective portion of the reticle membrane 22, 32. The subfields 22a, 32a are separated from one another on the reticle by boundary zones (e.g., item 25 in FIG. 7(a)) that do not define any portion of the pattern. Extending outwardly from the boundary zones 25 are support struts 23 that add substantial rigidity and strength to the reticle.
Each subfield 22a, 32a represents an area of the reticle that can be exposed at any one instant, and each subfield is typically approximately 1-mm square in size. Hence, on the reticle, the entire pattern to be transferred to a chip-sized area (a xe2x80x9cdiexe2x80x9d corresponding to a semiconductor chip) on the wafer is divided into a large number of, typically, 1-mm square subfields. The subfields are exposed individually. As the subfields are thus xe2x80x9ctransferredxe2x80x9d to the wafer, the respective images of the subfields are xe2x80x9cstitchedxe2x80x9d together contiguously to form the entire pattern in each die.
As shown in FIG. 7(c), during pattern transfer, the subfields 22a, 32a are scanned in a stepwise manner by the charged particle beam to transfer, to a xe2x80x9csensitizedxe2x80x9d substrate (xe2x80x9cwaferxe2x80x9d) 27, the respective pattern portions defined by the subfields. By xe2x80x9csensitizedxe2x80x9d is meant that the substrate 27 is coated with a material (termed a xe2x80x9cresistxe2x80x9d) capable of being imprinted with the projected subfield images. FIG. 7(c) clearly shows the xe2x80x9creductionxe2x80x9d or xe2x80x9cdemagnificationxe2x80x9d of the images that occurs during projection, and the xe2x80x9cstitching togetherxe2x80x9d of the subfield images on the substrate in a contiguous manner.
Reticles for CPB microlithography normally are manufactured from xe2x80x9creticle blanksxe2x80x9d that include the reticle membrane and the supporting struts. The lattice-like arrangement of the support struts defines intervening spaces on the reticle membrane in which the various subfields will be formed. A conventional process for making a reticle blank that includes a silicon membrane is shown in FIGS. 6(a)-6(f).
In a first step (FIG. 6(a)), a silicon substrate having a (100) surface orientation is prepared, and boron is diffused (e.g., by thermal diffusion or ion implantation) into one major surface of the substrate to form an xe2x80x9cactivexe2x80x9d silicon layer 12 (FIG. 6(b)). During a later etching step, the active silicon layer acts as an etch-stop layer. The active layer 12 is also destined to become the silicon membrane of a reticle formed from the blank, with the remainder of the silicon substrate 11 (i.e., everything except the active layer 12) being regarded as the silicon support portion 11a (FIG. 6(b)).
Next, a silicon nitride film 13 is formed over the entire outer surface of the silicon substrate 11 (FIG. 6(c)). A wet-etching mask 15 is formed by etching a pattern of xe2x80x9cwindowsxe2x80x9d (openings) 14 in the silicon nitride formed on the lower (in the figure) major surface of the substrate (FIG. 6(d)). Although only one window 14 is shown in the figure, normally a large number of such windows 14 are formed, corresponding to the number of subfields into which the reticle will be divided.
Next, the silicon substrate 11 (with etching mask 15) is immersed in an etching solution such as potassium hydroxide (KOH). The KOH solution wet-etches the silicon support portion 11a in the windows 14 not protected by silicon nitride. The etching solution removes silicon mainly in the depthwise direction. The etching rate drops abruptly when etching has reached the active silicon layer 12, thus stopping the depthwise etch (FIG. 6(e)).
If the boron concentration in the active silicon layer 12 is less than 2xc3x971019 atoms/cm3, no significant drop in etch rate will occur, and etching will proceed through the active silicon layer 12. Hence, to serve as an effective etch-stop layer, the concentration of boron in the active silicon layer 12 must be at least about 2xc3x971019 atoms/cm3.
In the last step, the silicon nitride etching mask 15 is removed to complete manufacture of the reticle blank (FIG. 6(f)).
The reticle blank shown in FIG. 6(f) comprises a silicon membrane 12b made of an active silicon layer diffused with boron, and silicon support struts 11b that provide support for the membrane 12b. To form a reticle from the reticle blank, a resist is applied to the membrane 12b of the reticle blank, and the prescribed pattern is written into the resist using an electron beam direct-write lithography system. The membrane 12b is then etched, with the patterned resist serving as the etching mask, to complete manufacture of a scattering-stencil reticle.
As described above, providing a diffused-boron concentration in the active silicon layer of at least approximately 2xc3x971019 atoms/cm3 provides an effective etch-stopping layer. However, whenever the active silicon layer is used as a reticle membrane in a reticle formed from the reticle blank, the greater the amount by which the boron concentration exceeds 2xc3x971019 atoms/cm3, the greater the tensile stress in the reticle membrane. High tensile stress in a silicon reticle membrane can cause distortion of a pattern from the reticle to a sensitive substrate. High tensile stress also can affect adversely the distribution of the boron concentration depthwise in the silicon membrane.
FIG. 8 shows a representative distribution of boron concentration depthwise in the silicon membrane of a conventional reticle blank. In FIG. 8, the ordinate (vertical axis) is boron concentration plotted on a logarithmic scale, and the abscissa (horizontal axis) is a depth dimension in the membrane plotted on a linear scale. On the abscissa, 0 xcexcm corresponds to the upper (in the figure) major surface of the membrane (i.e., the surface into which the boron is diffused). Measurements of boron concentration were performed by SIMS (secondary ion mass spectrometry) analysis.
From FIG. 8 it can be seen that the boron concentration is greatest just below the upper surface, and increases and then decreases with increasing depth. The variance in boron concentration through the thickness dimension is approximately 30%, as calculated from the difference of the lowest boron concentration from the highest boron concentration divided by the mean boron concentration (1.6xc3x971020 atoms/cm3). Corresponding to the profile of boron concentration, tensile stress near the top surface of the membrane (where boron concentration is high) is higher than in locations deeper in the thickness dimension. Hence, whenever a pattern to be transferred to a sensitive substrate is formed in the membrane, the membrane tends to exhibit distortion of its top surface (where stress is greatest). Such distortion also appears in the pattern as projected from the reticle.
Along with the trend toward integrated circuits having increasingly smaller circuit elements, the accuracy requirements demanded of reticles used for X-ray or CPB microlithography are much stricter than required of reticles used with conventional optical microlithography. I.e., the patterning and positioning accuracy requirements for X-ray and CPB microlithography is on the order of a few tens of nanometers. Consequently, phenomena such as membrane distortion due to a non-uniform depthwise distribution of tensile stress caused by a corresponding non-uniform boron concentration, the associated high tensile stress, and the attendant pattern distortion are now serious problems.
In view of the shortcomings of the prior art as summarized above, an object of the invention is to provide methods for manufacturing reticle blanks having reduced tensile stress and that are not subject to distortion caused by excessive tensile stress.
To such end, and according to a first aspect of the invention, methods are provided for manufacturing a reticle blank. In one embodiment of such a method, a silicon substrate is provided that comprises an active silicon layer on a major surface of a silicon support portion. At least on the silicon support portion, a wet-etching mask is formed that defines a prescribed pattern of openings corresponding to a desired pattern of windows. (The pattern of windows typically corresponds to an anticipated pattern of reticle subfields separated from one another by struts formed from remaining substrate.) The silicon support portion exposed in the openings is then wet-etched depthwise toward the active silicon layer. A protective film of an oxygen-impermeable material (desirably silicon nitride) is formed over the product of the wet-etching step, followed by a thermal anneal. Finally, the protective film is removed. The wet-etching mask can be removed before or after removing the oxygen-impermeable material.
The active silicon desirably is doped with boron to a concentration of 2xc3x971019 to 5xc3x971020 atoms/cm3 for optimal stress reduction while still allowing the active silicon layer to serve as an etching-stop layer. Further desirably, the concentration of boron through the thickness dimension has a variance of no greater than 1%.
The thermal annealing step desirably is performed at temperature range of 1000xc2x0 C. to 1200xc2x0 C. for a time ranging from 2 to 4 hours.
In another embodiment of a method according to the invention for forming a reticle blank, a silicon substrate is provided that comprises an active silicon layer on a major surface of a silicon support portion. At least on the silicon support portion, a wet-etching mask is formed that defines a prescribed pattern of openings corresponding to a desired pattern of windows. The silicon support portion exposed in the openings is wet-etched depthwise toward the active silicon layer to expose corresponding portions of the active silicon layer. A protective resist is applied to the exposed portions of the active silicon layer. The wet-etching mask is removed, followed by removal of the protective resist. A protective film of an oxygen-impermeable material is then formed over the entire product of the previous step, followed by thermal annealing. After annealing, the protective film is removed.
According to another aspect of the invention, methods are provided for manufacturing a reticle for charged-particle-beam microlithography. Typically, such a method comprises providing a reticle blank by a method as summarized above, followed by defining a pattern in or on the active silicon layer.
The invention also encompasses reticle blanks and reticles made according to a method as described herein.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.